The players need to have one of the "team socks" selected in the in-game edit player setting.
In teamgear.dbf under home uniform I set the color code value for the AUTOCOLOR field to 14343898 (white). The SOCKCOLOR value should be set to 0.
And for the away uniform, I set the color code for AUTOCOLOR field to 1644825 (black). The SOCKCOLOR value should be set to 1.
And presto, it seems to work. Here's Philly:

I haven't checked to see how this affects teamgear accessories, I imagine it will, but for the time being, I'm happy to have white/black home and away socks...